This invention was made under a Joint Research Agreement between NTN Corporation, Japan, Asahi Glass Matex Co. Ltd., Japan, and Mitsubishi Rayon Co. Ltd., Japan. The term of the Joint Research Agreement is from Dec. 1, 2000 to Mar. 31, 2002.
1. Field of the Invention
The present invention relates to a fiber reinforced plastic pipe for use such as with a propeller shaft or a drive shaft which constitutes part of the power transmission system of a vehicle. The present invention also relates to a power transmission shaft incorporating the fiber reinforced plastic pipe.
2. Description of the Related Art
Those shafts that constitute the power transmission system of a vehicle include a propeller shaft for transmitting power from the gearbox to the reduction gear, and a drive shaft for coupling the engine and the hub joint. Any of the shafts is provided at the ends thereof with universal joints, and constructed to respond to variations in length and angle resulting from changes in relative position between the gearbox and the reduction gear or between the engine and the hub joint.
FIG. 8 is a view illustrating an example of a power transmission shaft, showing the overall view of a propeller shaft 1. The propeller shaft 1 is provided with stub shafts 3 and 4, a type of metal joint elements, which are jointed to the both ends of an intermediate shaft 2. The stub shafts 3 and 4 mate with the joint inner ring of constant velocity joints 5 and 6 by means of splines or serrations. FIGS. 9a and 9b are views illustrating another example of a power transmission shaft, showing an intermediate shaft of the drive shaft. FIG. 9a illustrates an intermediate shaft 7 provided with no dynamic damper, and FIG. 9b illustrates an intermediate shaft 8 provided with a dynamic damper 9 for preventing vibrations.
It has been a conventional process to employ hollow or solid steel shafts for the intermediate shaft 2 of the propeller shaft 1 and the intermediate shafts 7, 8 of the drive shaft. From the viewpoint of flexural rigidity, an elongated intermediate shaft that constitutes the power transmission shaft needs to be provided at the intermediate portion thereof with a support bearing or a dynamic damper for preventing vibrations. Accordingly, improvements have been required for the shaft in weight and cost, and thus it has been difficult to provide an elongated shaft.
In view of these improvements, it has been suggested to employ a hollow pipe formed of a fiber reinforced plastic (hereinafter referred to as the FRP pipe) and having a high flexural rigidity in combination with a metal pipe. This improves the flexural rigidity of the intermediate shaft to eliminate the support bearing or the dynamic damper for preventing vibrations at the intermediate portion, thereby reducing the shaft in weight and cost and making it possible to provide an elongated shaft.
For example, the FRP pipe can be reduced in thickness and increased in diameter by pultrusion process. By pultrusion process, a plurality of fiber bundles are spun and hardened continuously while being aligned in the longitudinal direction, thereby making it possible to efficiently manufacture a pipe-shaped molded product having a uniform cross-section. To combine the FRP pipe obtained by the pultrusion process with a metal pipe, such a typical technique has been processed in which the FRP pipe is mechanically press-fitted into the metal pipe.
That is, it is conceivable that the FRP pipe is inserted into the metal pipe and then the outer circumference of the metal pipe is reduced in diameter by plastic-working in order to fix the FRP pipe to the metal pipe. Alternatively, to fix the FRP pipe to the metal pipe, an adhesive can be injected into the gap between the metal pipe and the FRP pipe. However, in a case where the FRP pipe and the metal pipe are formed in the shape of a simple cylinder and have a tolerance of size, respectively, it is necessary to match the dimensions with each other to control subtle press-fitting or press-fit pressures in the process, thereby making the assembly extremely complicated.
In contrast to this, using an FRP pipe having a slit in the axial direction would eliminate the aforementioned subtle matching for press-fitting and reduce the press-fit pressure. In other words, the FRP pipe can be reduced in diameter in the circumferential direction when the FRP pipe is incorporated into the metal pipe. This allows for a plastic deformation by the width of the slit, thereby making the assembly considerably easier in comparison with a case where a cylindrical pipe having no slit is press-fitted into the metal pipe.
Furthermore, suppose that the FRP pipe with a slit is formed to have an outer diameter greater than the inner diameter of the metal pipe, so that the apparent outer diameter of the FRP pipe is less than the inner diameter of the metal pipe when the FRP pipe is elastically deformed by the width of the slit in the circumferential direction. In this case, after the assembly, the FRP pipe will expand inside the metal pipe back to the original outer diameter provided upon its formation. This elastic recovery force serves to press the FRP pipe against the metal pipe, thereby making it possible to securely fix the FRP pipe to the metal pipe.
However, in a case where upon inserting the FRP pipe into the metal pipe, the FRP pipe is so damaged due to cracks as not to make use of its practical performance, the effectiveness of the aforementioned various techniques is ruined, making it difficult to employ the FRP pipe for use with the power transmission shaft.
To form the FRP pipe by pultrusion process, it is a common process to draw all fiber bundles being aligned in the longitudinal direction of the pipe in order to make full use of the mechanical properties of the fibers that are used for the FRP pipe. However, an FRP pipe which is reduced in thickness and increased in diameter by pultrusion process makes such a problem noticeable that the FRP pipe is sensitive to crushing force exerted from the circumference, thereby causing longitudinal or other cracks to readily develop.
Furthermore, when the FRP pipe with a slit is elastically deformed by the width of the slit and thus reduced in diameter, the maximum tensile stress occurs on an outer surface of the pipe, the outer surface being opposite to the slit with respect to the center of the pipe. This tensile stress increases in proportion to the width of the slit of the FRP pipe, thereby increasing the possibility of occurrence of cracks. In particular, reinforcement fibers spun being aligned in the longitudinal direction of the pipe may easily raise the problem that longitudinal or other cracks are also caused by the stress developed due to thermal expansion in addition to the stress caused by an external force.